The present invention relates to an image recording process wherein latent images are: (1) formed in certain recording elements containing a reducible metal compound by flowing a small, imagewise pattern of electrical current therein and (2) subsequently dry processed to produce a visible image.
U.S. Pat. No. 3,138,547, issued June 23, 1964, in the name of B. L. Clark discloses that visible images can be formed in certain electrosensitive recording materials comprising a continuous electrically conductive layer and a contiguous layer composed of at least one particulate metal compound capable of electrical reduction in situ. Recording is effected by passing through the recording material an imagewise current of sufficient magnitude to reduce the particulate metal compound in the dry state. The free metal produced provides a visible image which varies in intensity in accordance with the current passed.
A drawback of the recording process disclosed by Clark is that it incorporates no gain or amplification. For each reduction event leading to an increase in density of the final image, an additional quantity of electronic charge flowing through the recording material must be provided. Thus, relatively high current densities must be provided in order to produce a visible image in a reasonable period of time.
In the above-referenced U.S. application Ser. No. 492,814, incorporated herein by reference, an electrographic recording process is disclosed which forms a latent image in certain charge-sensitive, dry processable recording materials by passing a relatively minute amount of electrical charge through the material in an imagewise pattern. The latent image is then amplified and rendered visible by heating the entire recording material substantially uniformly. Since the charge exposure is required only for latent image formation, the magnitude of such charge exposure (approximately 1 microcouloumb/cm.sup.2) has been found to be several orders of magnitude less than that required by the prior art dry direct image recording processes. Another advantage of the new process is its ability to record information emanating from a variety of exposure sources including tungsten lamps, xenon lamps, helium-neon laser beams, infrared radiation and X-ray radiation by the appropriate selection of a photoconductor which serves as an opto-electrical transducing device. Any source of radiation to which the photoconductor is responsive may be used as the exposure source, provided that the dynamic resistance of the photoconductor closely matches the dynamic resistance of the recording material in the operating voltage range of the process.
When X-ray radiation is used as the exposure source, it is usually desirable for health and safety reasons to limit the quantity of such X-ray radiation to the lowest level required to produce a copy of a desired density or contrast.